The present invention provides catalyst systems for use in the polymerization of olefins, especially propylene and copolymers of propylene, that produce a polymer with a broad or multimodal molecular weight distribution. These catalyst systems include a mixture of at least two different chiral, stereo-rigid metallocene catalysts.
The use of metallocene catalysts in the polymerization of olefins is known in the art. German Patent Application No. 2,608,863 discloses a catalyst system for the polymerization of ethylene which system consists of a bis(cyclopentadienyl) titanium dialkyl, an aluminum trialkyl and water. Similarly, German Patent Application No. 2,608,933 discloses a zirconium metallocene of the formula (cyclopentadienyl).sub.n ZrY.sub.4-n, wherein Y represents R, CH.sub.2 AlR.sub.2, CH.sub.2 CH.sub.2 AlR.sub.2, or CH.sub.2 CH(AlR.sub.2).sub.2 and where R is an alkyl or metallo alkyl and n is an integer within the range 1-4. This catalyst is described as being useful in the polymerization of ethylene.
Metallocene catalysts are known to be useful in the copolymerization of ethylene and other alphaolefins. U.S. Pat. No. 4,542,199 to Kaminsky, et al. discloses a catalyst system that comprises a catalyst of the formula (cyclopentadienyl).sub.2 MeRHal, in which R is a halogen, a cyclopentadienyl group, or a C.sub.1 -C.sub.6 alkyl radical; Me is a transition metal, in particular zirconium, and Hal is a halogen, in particular chlorine. The catalyst system also includes an alumoxane of the formula Al.sub.2 OR.sub.4 (Al(R)-O).sub.n for a linear molecule and/or (Al(R)--O).sub.n+2 for a cyclical molecule in which n is a number from 4-20 and R is a methyl or ethyl radical. A similar catalyst system is disclosed in U.S. Pat. No. 4,404,344.
Metallocene catalysts particularly useful in the polymerization of propylene and higher alpha-olefins are disclosed in European Patent Publication No. 0185918. This Publication discloses a zirconium metallocene catalyst that includes a bridge between two cyclopentadienyl rings. The bridge is described as being a linear hydrocarbon with 1-4 carbon atoms or a cyclical hydrocarbon with 314 6 carbon atoms.
Other metallocene catalysts are also disclosed in co-pending U.S. applications Ser. Nos. 034,341 (now abandoned) and 034,472 (now abandoned) both of which are inventions by the present inventor and are assigned to the same assignee. Application Ser. No. 034,472 (now abandoned) describes a method for varying the melting points and molecular weights of polyolefins by varying the bridge and other substituents on a metallocene catalyst. Application Ser. No. 034,341 (now abandoned) discloses a catalyst system comprising a stereo-rigid hafnium metallocene catalyst in combination with an aluminum compound. The catalyst is described by the formula R"(C.sub.5 R'.sub.m).sub.2 -HfQ.sub.p wherein R" includes a bridge between the two (C.sub.5 R'm) rings. These hafnium metallocene catalysts are more stereo-specific and produce polymers with higher molecular weights than previously obtainable with metallocene catalysts.
The metallocene catalyst systems described above typically produce a polymer product with a relatively narrow molecular weight distribution (MWD). This distribution, as defined by Mw/Mn, is usually within the range of 2-4. For some polymer applications, it is desirable to have a narrow MWD; for others, a broad MWD is desirable.
U.S. Pat. No. 4,530,914 discloses a catalyst system for the polymerization of ethylene to polyethylene having a broad MWD which may be bimodal or multimodal. The catalyst system comprises at least two different metallocenes that have different rate constants for the propagation and termination of ethylene polymerizations. The disclosure is limited to the production of ethylene, and the disclosed catalyst system does not address the stereochemical problems of making useful polypropylene and other higher alpha-olefins.
The present invention provides a catalyst system that may be used to polymerize almost all olefins, and not just ethylene, and to produce a polymer product with a broad MWD. The MWD may be tailored to a desired level by varying the substituents of the catalyst system.